Comment on NanoVNA vs. Loop antenna SWR testing [Question]
fullsquare@awful.systems 5 days ago
you can get away with very inefficient antennas on HF reception, so i wouldn’t take SDR reading very seriously. (atmospheric noise dominates all noise, so amplification will get you useful signal with amplifier not introducing significant noise on its own. reverse is true on vhf, and especially on uhf and up)
resonance should happen no matter what power level. do you mean SWR 50 or 50 ohm? i’m not even sure if nanovna can measure SWR that high. it sounds like you have a short or open somewhere it shouldn’t be? you need to calibrate it after changing tested frequency range, have you done that? (calibration can be saved). at the vhf-ish frequencies, it would make sense that your loop becomes full wave or even larger. circular loop has impedance of some 100 ohms, but you have capacitor at the ends of it so it’s gonna be different
with magloops, with set size of loop, tune is via changing capacitance, match is via changing position (closer or further from loop, tilt away from plane of loop), shape or size (cross sectional area) of feed loop, you can match it exactly this way. coax stub can be lossy, if it’s just 4W then probably not a problem but with higher powers check if it’s not overheating
einfach_orangensaft@sh.itjust.works 5 days ago
Thank you for your answer!
I think i found the root of my problem, the scale settings on the NanoVNA where wrong, to a point where the super slim SWR dip of the Loop was just not drawn on the graph for lack of data points at this point. I changed the scale and was able to see the dip, then tune the coax stub to the frequency i wanted. Now it claims to have a SWR of 3.8 near the frequency i want to use:
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But i am still a bit confused, the yellow number left of the SWR reading, i assume that is the scale? or does it mean a SWR of 6.125:3.8?
LH0ezVT@sh.itjust.works 5 days ago
Second number should be SWR, first one scale (value change per horizontal box). So that would be 3.8 SWR, which corresponds to about half a box of 6.1 - 1, because the lowest you can get is 1.
fullsquare@awful.systems 5 days ago
idk how you have done that, maybe i have older version but for me this marker just reads CH0 SWR 1.00/(value)
you can pull up a smith chart, this will tell you whether impedance is too low or too high, since it’s still not matched at resonance
LH0ezVT@sh.itjust.works 5 days ago
All good points, especially calibration. Without, it still gives you a general idea, but it’s more a “meh looks OK” than a serious value.
Also, as fullsquare said, take a picture of yhe smith chart / impedance display as a second check, if it shows something like alot + j toomuch, you know your SWR is actually bad, and you know what direction you need to go for (add more capacity, inductance, or some resistive transformation).
fullsquare@awful.systems 5 days ago
i’ll add that in a way SWR chart is more resistant to misuse, because if nanovna is calibrated with wrong length of 50 ohm feedline, or without feedline at all, then smith chart will be rotated by angle depending on difference in length of that feedline, while SWR chart should look the same. for example, if real part of impedance at resonance is too low (ex. 20 ohm), and feedline is quarter wavelength different from what nanovna was calibrated with, then impedance will be still real but too high (ex. 125 ohm), while SWR chart should look the same (1:2.5 SWR minimum) (barring losses in feedline). (this works the same way as quarterwave long feedline impedance matching scheme). for different feedline length differences (non-multiple quarterwave) impedance will be complex at antenna resonance. this problem is avoided by calibrating nanovna with feedline
einfach_orangensaft@sh.itjust.works 4 days ago
Here is the measurement with smith chart:
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There is 2m of BNC RG58 coax between the antenna and the NanoVNA. I calibrated the NanoVNA without the coax line because i was under the impression the line to the antenna is considered part of the radiating system and hence should not be calibrated out. But here is the reading with the coax line to the antenna calibrated out:
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